We have some pcb in which contains an MCU ,and some WiFi module .
Usually at start, the board takes 70-100mA with 3v3 .
It gets its current from the UNO board connected to a mac.
It works great like that .(if you wonder how the UNO can get this current,me too)

Problem starts when we try to measure current. If we take the fluke device, and put it in serial to the voltage of the pcb to measure current, what happens is that the MCU starts to get resets, and the WiFi module voltage is drops .nothing is works than.

We have tried to change the current scale on the Amper-Meter , from 20m to 20A.
Nothing helped.

*if we measure current of other stuff that consumes lest current, it DOES measure and show the right current.
The problem occurs only with the pcb that contains a WiFi module that takes 70-100mA.

2 Answers
2

This is the "burden voltage" of an ammeter - when a 'shunt resistor' is used to measure the voltage drop due to current flow, then the voltage seen by the circuit being powered is lower. Multimeters, because of their wide sensing range, tend to have relatively high shunt resistors, which causes the circuit being powered to see significantly less voltage, and so some things don't work right.

I'd recommend putting a very low value resistor in series with the 3.3V rail that's powering the wifi module (say, 100mohm), and then measure the voltage across that resistor & do the Ohm's Law calculation to work out the current.

\$\begingroup\$thanks, i could see that putting it in 20A scale, will than enable the measurement,but i wouldn't see anything than because the currents are low..\$\endgroup\$
– CurneliousJun 28 '15 at 9:38

\$\begingroup\$Either get a low-value precision resistor (to make the maths easier; you need to know the resistor value as precisely as you want your measurement to be, and say one that's 0.1049 ohms means it's harder to do in your head), or you can use something like eevblog's microcurrent which has the precision resistor and an op-amp which does the maths for you.\$\endgroup\$
– Pete KirkhamJun 28 '15 at 11:14

2

\$\begingroup\$@Curnelious, check out Dave Jone's uCurrent; it was designed for precisely this purpose. On one side you inject the current (with very, very low shunt resistance) and on the other side you measure a proportional voltage.\$\endgroup\$
– sherrellbcJun 28 '15 at 14:24

Check your multimeter documentation, if it is decent it will list the burden voltage for each current range. You mention an unspecified Fluke multimeter, I've got a Fluke 87V and on its manual you will find this table:

The same Dave Jones also built an add-on for multimeters to cope with small currents measurements and high burden voltage: the μCurrent. Note: although he sells those devices on his site, it is Open-Source Hardware, so you may build your own. On his youtube channel or on his site you'll find additional videos on that device (both on its usage and on how he designed/built it).

BTW, note the funny-looking way they usually specify the burden voltage: in that table above, for example, they say 1.8mV/mA instead of simply 1.8Ω or also 100μV/μA instead of 100Ω. They want to underline that for each mA your load draws the multimeter causes an additional 1.8mV voltage drop.

Moreover, see how the burden voltage increases on low-current ranges! That multimeter is an industrial multimeter, so it is not designed with low-voltage+low-current applications in mind (in industrial environments often low-current circuits are relatively high-voltage, so a 100mV/mA burden voltage is not a real concern).

This is common also for cheap or middle-range multimeters usually, since measuring very low currents with a current shunt and low burden voltage means that the amplifiers in the multimeter must be able to cope with voltages in the 1mV range or less. This requires precision, low-drift, low-noise amplifiers and accurate design of input stages, which adds to the overall cost of the multimeter.